All-optical wavelength converters based upon difference frequency generation (DFG) in periodically-poled materials have been described by Chou et al., Optics Letters, vol. 23, pp. 1004-1006, July 1998; and Chou et al., Optics Letters, vol. 24, pp. 1157-1159, August 1999, to which reference is periodically made.
An optical parametric amplifier (OPA) is a more general case of a DFG. From a given pump wavelength, an OPA produces two outputs of different wavelengths whose energies sum to equal the energy of the pump, such as described in U.S. Pat. No. 5,181,211, entitled “Eye-Safe Laser System,” which issued to Kasinski et al. For example, a 730 nm pump can generate a signal at 1310 nm and an idler at 1648 nm. FIGS. 1A and 1B illustrates the arrangement and operation of an OPA. In the block diagram of FIG. 1A, the OPA is formed by two parts, a DFG portion and a second harmonic generation portion (SHG) portion, which doubles an ITU (International Telecommunications Union) pump frequency to an equivalent wavelength of λp/2 from which the DFG portion generates an amplified input signal and idler. One can think of an OPA as follows—in frequency space, the pump at frequency ωp forms a “mirror” at frequency ωp/2, and the signal and idler are sidebands or “reflected images” equally spaced on either side of the central pump frequency ωp/2, as depicted schematically in FIGS. 1A and 1B. In wavelength space as shown in FIG. 1B, the signal and idler wavelengths “mirror” around pump wavelength λp.
For telecommunications applications, in which CW (continuous wave) or weakly modulated signals are used without significant power, the ideal medium for the OPA is a periodically poled substance, such as periodically poled lithium niobate (PPLN), as described by Chou et al., Optics Letters, vol. 23, pp. 1004-1006, July 1998; and Chou et al., Optics Letters, vol. 24, pp. 157-1159, August 1999. Alternate materials include periodically poled lithium tantalate, or a periodically grown semiconductor material, such as GaAs or InGaAs. The periodic poling achieves non-critical phase matching for a wide range of wavelengths, thereby maximizing the nonlinear gain for even weak CW signals.
The devices described in these papers demonstrate broad acceptance of input signals enabling conversion of a wide range wavelengths, even simultaneously. However, these prior art device are limited to at most four possible wavelength shifts within a single device, and for each additional shift increasing losses are suffered (eg., 2× loss for 2 shifts, 4× loss for 4 shifts). These renders the possibility of an any-to-any wavelength converter remote. Also, the amount of shift varies for a given pump, depending upon the spectral separation of the input signal and pump. This prevents shifting a single channel to any other channel. Even tunable lasers do not enable such an any-to-any device, since only 4 shifts could be allowed in a given chip, far less than the 80 ITU channels that exist today.
In contrast, the present invention solves these problems, and in a sense, reverses what has been done in the prior art to create new functionality and performance from nonlinear optical wavelength converters.